A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to determine a property of the substrate, such as its alignment, a beam is reflected off the surface of the substrate, for example, at an alignment target or mark, and an image of the reflected beam is created on, for example, a camera. By comparing one or more properties of the beam before and after it has been reflected off the substrate, one or more properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with one or more known substrate properties.
Scatterometry is an active field of research where optical techniques are used to measure subwavelength features of an object. In an embodiment, the apparatus used to measure the subwavelength features is an in-line metrology tool. This metrology tool detects a reflected beam that has been reflected from the surface of a substrate and more specifically from a specific target on the substrate, and from the reflected beam and its different diffraction orders, reconstructs the shape of the target on the substrate. The reconstructed target can then be compared with other data or with an ideal structure and this information may be fed back to the lithographic apparatus in order to correct for defects that might be occurring in the exposure stage. Alternatively or additionally, the information may be fed forward to one or more other processes that are carried out on a substrate in order to compensate for any errors or deviations in the exposure step.
A part of the detection and reconstruction is a numerical algorithm that calculates the diffraction pattern from an arbitrary (or model) target profile. Currently, RCWA (rigorous coupled wave analysis) is used for this purpose. RCWA is a relatively fast algorithm, but may lose its speed on particular target types. This can be a disadvantage for robust calculation of line profiles within an acceptable amount of time. If the calculation of the reconstruction of the target takes too long, a large gap of time is created between a substrate being exposed and any errors being found and compensated for. Ideally, any errors should be found as the substrate is on its way to the next process so that the information can be fed forward in time to correct for errors in the same substrate that has just been exposed and measured.